While it has been known to combine silicone emulsions with natural rubber latex in minute amounts, on the order of 0.025 parts by weight of the latex, for purposes of defoaming or reducing adhesion characteristics of the latex foam, combinations of larger amounts have not been successful inasmuch as they are merely mixtures of the components which tend to separate or "bleed out" of the latex with migration to the surface. Such bleeding presents problems in the manufacture and use of multi-layered products such as de-lamination, local areas of surface tack and in the case of Foley catheters, weaknesses in the inflatable balloon.
Natural rubber latex compounds suitably employed for the manufacture of high quality surgical tubing and catheters comprise 96% rubber by weight and 4% of other ingredients such as curatives, fillers, anti-oxidants and stabilizers. These formulations have not heretofore accepted even small amounts of silicone, i.e., 3 to 5% by weight per hundred parts of rubber, and produced finished products having satisfactory functional characteristics.
One U.S. patent, No. 3,962,519, does provide for incorporating silicone compounds in natural and synthetic rubber and relies upon the bleeding or migration of the silicone compound to the external surface of the instrument to provide resistance to unwanted adhesion and water repellency. The patent discloses employment of silicone compounds having a molecular weight of more than 90,000 and states that the amount present can range from about 0.1 to 10% by weight based upon the weight of the rubber. Of course, due to the migratory behavior of the silicone, incorporated according to the disclosure of this patent, relatively high amounts of silicone cannot be employed without causing separation of the silicone from the rubber latex and concommitant failure of the product. Notwithstanding the disclosure and claims of this prior patent, I have not found it possible to incorporate more than about 1% of a silicone compound of the type disclosed therein and form a satisfactory product.